Come 6pm, the tourists are joined by middle-class country bumpkins up for a 24-hour treat with their spouse, children left behind with the baby-sitter. Whether it is pre-dinner drinks or pre-theatre dinner, they head for the Ritz.
"We're having dinner here before Phantom," whispers Mrs Norma Kalbraier. She and her friend Rosemary Penkethman are housewives on a runner from Ipswich, Suffolk. Sassy in silk, they aren't staying at the Ritz, of course: nobody except for the tourists, fading rockstars and newly weds does that. It's classier "just to pop in for dinner".
True, Mayfair habitues keep well clear of this early evening Piccadilly charade. After all, there's no rush to go to the theatre on Friday - not if one has already been on Monday or Tuesday. To genuine Mayfairites, the concept of "Friday night out" is an invention of the proletariat, a necessary evil stemming from the pattern of the working week.
"We wouldn't go normally go out on a Friday," explains Deborah Williams, up from the country to the family flat to take daughter Amy, 10, to the ballet at Covent Garden. "If I come up to London to go out with my husband, it's earlier in the week. We might eat at Claridges or the Savoy after the theatre. Fridays are a good night to take Amy, because of school."
Mr Bishop, 57, a freelance marketing publisher from Knightsbridge, admits somewhat shamefaced that he would be dining at the Ritz "only because it is a very special occasion. It's my wedding anniversary; I'm taking my wife and daughter out to dinner. My usual haunt is Ronnie Scotts or restaurants around Knightsbridge."
Really cool Mayfair residents don't go out at all on Fridays. Alex Bailey, who co-owns Lady Daphnes, a very upmarket hardware store in Sloane Street, has no intention of straying from the television. "I'm going home to put my feet up," he announces, clutching bundles of cheap wine outside the Oddbins near Shepherds Market.
As the evening wears on, the picture becomes clear: Mayfair toffs stay in or dine with each other on Friday nights. Only proles and tourists hit the town. "I'm going to Quagliaaaarnoes," mispronounces an American at the top of his voice. An elderly couple in black tie, heading for Brooks's, la creme de la creme of St James's, cringe.
Still, appearances can be misleading. Boodles, instead of being filled with the suited types to be found in all the other St James's clubs, hasopened its doors this Friday night to a school's old boys' reunion. There is something faintly comical about the sight of two school minibuses parked outside the club's sober entrance in between all those Jaguars and Mercedes.